This invention relates to novel compositions of matter useful in the selective removal of immune complexes from serum. More specifically, this invention relates to an immunosorbent material comprising plural polypeptide domains designed to bind immune complex with greater affinity than free, circulating immunoglobulin.
Immune complexes have been implicated in the pathology of a number of human disease states. Indeed, the serum of many individuals with autoimmune disease, neoplastic disease, acquired immune deficiency syndrome, and some infectious diseases can be demonstrated to contain high levels of circulating immune complexes. Such complexes have been hypothesized to mediate a variety of immunologic effector functions. Removal of the complexes from circulating blood is expected to have therapeutic benefit.
U.S. Pat. No. 4,614,513 describes a method and apparatus for removing "immunoreactive substances" from blood comprising at least "components of Protein A". Protein A is a cell wall component of most strains of Staphylococcus aureus which has the capacity to bind specifically to the Fc region of a number of immunoglobulin species. The native protein is partially buried in the cell wall via its hydrophobic N-terminal segment which consists of about 150 amino acid residues. The remainder of the molecule consists of five highly homologous domains designated E, D, A, B, and C, which are consecutively arranged along the polypeptide chain, each having a molecular weight of approximately 7000 daltons. Each domain has the capacity to independently bind one Fc region of an immunoglobulin with apparently equal affinity. This binding interaction has an association or binding constant (K.sub.a) of approximately 5.times.10.sup.7 M.sup.-1, which varies slightly with the pH of the buffer and with the species, class, and subclass of the immunoglobulin. However, Protein A is able to bind only two immunoglobulin molecules at one time, presumably due to steric constraints.
The binding of Protein A to the Fc region of an immunoglobulin has no significant effect on the affinity of the immunoglobulin for its antigen. Protein A from native and recombinant sources accordingly is useful as an immunosorbent for a variety of diagnostic and basic research applications. See European Patent No. 83306500.6 and International Patent Application Nos. PCT/SE/83/00297 and PCT/SE83/00298. These applications disclose "Protein A-like" molecules with substantially the same "Protein A-like binding" or increased "IgG-binding activity".
Unrecognized in the disclosure of U.S. Pat. No. 4,614,513 is the constraint on the method there disclosed that Protein A binds both free and complexed immunoglobulin. Thus, Protein A cannot be used practically as a therapeutic reagent to selectively remove immune complexes in the presence of uncomplexed, soluble immunoglobulin.
It is an object of this invention to provide a novel immunosorbent material which is useful, for example, in the selective removal of immune complexes from blood or serum. Another object is to provide a DNA sequence encoding this immunosorbent polypeptide, and to provide a method for the removal of immune complexes from serum in the presence of free, circulating immunoglobulins.
These and other objects and features of the invention will be apparent from the following description, drawing, and claims.